1. Field of the Invention
The present invention relates to a thin film transistor substrate constituting a display, and a manufacturing method for the same.
2. Description of the Background Art
A TFT active matrix substrate (hereinafter referred to as “TFT substrate”) employing a thin film transistor (hereinafter referred to as “TFT”) as a switching element is, for example, used for an electro-optic apparatus such as a display using a liquid crystal (hereinafter referred to as “liquid crystal display”).
As for the electro-optic apparatus using the TFT substrate, there have been demands for improvement in display performance such as higher definition and higher product quality, as well as a demand for reduction in manufacturing process to efficiently manufacture these in quest of low cost.
A back channel type TFT has hitherto been in use as the switching element of the TFT substrate for the liquid crystal display. Typically, amorphous silicon (Si) is used as a semiconductor active layer in the back channel type TFT for the liquid crystal display, and a structure called a reverse stagger type has been adopted thereto. These TFT substrates are manufactured normally after undergoing four or five times of photolithography steps.
Meanwhile, in response to the foregoing demand for reduction in manufacturing process, Japanese Patent Application Laid-Open No. S64-35529 (Japanese Patent Application Laid-Open No. H1-35529), Japanese Patent Application Laid-Open No. 2004-281687 (Japanese Patent No. 4522660) and the like disclose a manufacturing method where the photolithography steps are reduced to three times by means of a structure called a stagger type being different from the reverse stagger type.
Further, in response to the demand for improvement in display performance, a TFT has been vigorously developed in recent years, the TFT using as the active layer the oxide semiconductor that indicates higher mobility than that of conventional Si (Japanese Patent Application Laid-Open No. 2004-103957 (Japanese Patent No. 4164562), Japanese Patent Application Laid-Open No. 2005-77822, “Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors”, Kenji Nomura, et. al., Nature 2004, Vol. 432, pp. 488-492).
As an oxide semiconductor material, research and development have been proceeded with focuses on a zinc oxide (ZnO) type, an IGZO type obtained by adding gallium oxide (Ga2O3) or indium oxide (In2O3) to zinc oxide (ZnO), or some other type.
The oxide semiconductor material as thus described has advantages of being etchable by a mild acidic solution such as oxalic acid or carboxylic acid, and being pattern-processed with ease. However, the oxide semiconductor material is also dissolved with ease by an acidic solution normally used at the time of etching a typical metal film (Cr, Ti, Mo, Ta, Al, Cu or an alloy of these) for use in a source electrode and a drain electrode of the TFT.
Accordingly, in the case of manufacturing the TFT using the oxide semiconductor as the active layer, for example as disclosed in Japanese Patent Application Laid-Open No. 2008-72011, the TFT is manufactured such that, even when a new element for improvement in resistance of the oxide semiconductor to a medical solution is added or film thicknesses of an oxide semiconductor film or a metal film are rightsized to remove the metal film by etching, the oxide semiconductor film does not disappear and remains.
The liquid crystal display using the TFT substrate is configured by bonding the TFT substrate with a counter substrate opposed thereto and injecting a liquid crystal between these substrates. On the TFT substrate, a TFT and a pixel electrode are arranged in an area surrounded by a plurality of scanning lines (gate lines) in a horizontal direction and a plurality of signal lines (source lines) orthogonal to these lines, to form a matrix shape. A counter electrode supplied with a constant voltage is formed on the counter substrate, and a liquid crystal capacitance is formed by liquid crystal held between the counter electrode and the pixel electrode.
A scanning signal is sequentially applied to the scanning line on the TFT substrate every horizontal scanning period, and further, a predetermined signal voltage is sequentially applied to the signal line (source line) in a vertical direction, to apply a display voltage to the corresponding pixel electrode so as to form an image. At this time, a predetermined charge is held in the liquid crystal capacitance over a frame period of horizontal scanning, thereby to realize display of a flat image for one frame. However, when the charge held in the liquid crystal capacitance fluctuates, a phenomenon of degradation in display quality such as flicker occurs. Hence a configuration has been adopted where, other than the liquid crystal capacitance, an auxiliary capacitance is formed in the pixel electrode on the TFT substrate to alleviate fluctuations in charge so as to prevent deterioration in display quality.
However, in the conventional manufacturing method for the TFT substrate where the photolithography steps are reduced to three times, the auxiliary capacitance is difficult to form. For example, as for the TFT substrate manufactured by the manufacturing method disclosed in foregoing Japanese Patent Application Laid-Open No. S64-35529 (Japanese Patent Application Laid-Open No. H1-35529), the configurations of the TFT and the pixel electrode are described (FIGS. 1 and 5), but the configuration of the auxiliary capacitance is not described.
Further, as for the TFT substrate manufactured by the manufacturing method disclosed in foregoing Japanese Patent Application Laid-Open No. 2004-281687 (Japanese Patent No. 4522660), the configuration of the auxiliary capacitance is described, but this auxiliary capacitance is configured in such a form that the counter electrode opposed to the pixel electrode uses a part of a scanning line layer via an insulating film (FIGS. 28 and 29). For this reason, there remain a problem of a small degree of freedom in designing a capacitance value of the auxiliary capacitance, and further a problem of a small margin with respect to deterioration in display quality such as flicker due to the possibility for fluctuations in capacitance value.
Moreover, for example among the configurations of the TFT substrates disclosed in Japanese Patent Application Laid-Open No. S64-35529 (Japanese Patent Application Laid-Open No. H1-35529) and Japanese Patent Application Laid-Open No. 2004-281687 (Japanese Patent No. 4522660), there is also a configuration where a light shield film such as the metal film is not formed between a foundation substrate and a semiconductor material. There has thus been a problem in that in the case of using conventional Si as the semiconductor material, when back light is incident from the rear surface of the substrate for the purpose of image display, a photo carrier is generated in a channel part of the TFT to cause deterioration in ON/OFF ratio of TFT switching characteristics, and degraded contrast of the display image, a cross talk and inconsistencies in display are generated to cause deterioration in display quality.
Further, although Japanese Patent Application Laid-Open No. 2004-281687 (Japanese Patent No. 4522660) also discloses the configuration of the TFT substrate formed with a light shield film between the substrate and the semiconductor film (FIG. 31), this causes addition of a new step of forming the light shield film.
This problem can be solved by use of an oxide-system semiconductor material in place of conventional Si as the semiconductor material. That is, the oxide semiconductor has high mobility and is influenced in a small degree by generation of the photo carrier due to light as compared with Si, thereby allowing suppression of deterioration in display quality.
However, in the case of a TFT substrate using such an oxide semiconductor, as described above, it is difficult to perform selective etching by means of a medical solution with a conventionally known general metal film which becomes lines and an electrode material. There have thus been problems of reduction in process margin for manufacturing, deterioration in performance of the oxide semiconductor TFT, and the like.